Inclusion Criteria:

• Male or female whose age on the day of randomization falls within one of the two age cohorts: Cohort 1: 5 to <24 months (reached their 5th month birthday but not yet reached their 2nd year birthday); Cohort 2: 1 to < 3 months (>28 days of age and not yet reached their 3rd month birthday)
• Cohort 1 only: Subject is seronegative to RSV at screening as determined by ELISA; or the legal representative is willing to provide access to data documenting that the subject was screened for another MedImmune trial after written informed consent was obtained, and that the subject is seronegative to RSV within 21 days prior to randomization into MI-CP147 as determined by ELISA at MedImmune
• Subject was the product of normal full term pregnancy (defined as 36-42 weeks gestation)
• Subject is in general good health
• Subject's legal representative is available by telephone throughout the trial period
• Written informed consent and HIPAA authorization (if applicable) obtained from the subject's legal representative
• Subject's legal representative is able to understand and comply with the requirements of the protocol as judged by the investigator
• Subject is available to complete the follow-up period, which will be 365 days after randomization
• Subject's legal representative is willing and able to bring the subject to the study site for evaluation of respiratory illness in accordance with the protocol

Exclusion Criteria:

• Any fever (≥ 100.4°F [≥ 38.0°C]), regardless of route within 7 days prior to randomization; rectal temperature measurement is strongly preferred for accuracy; temperature measurement by other methods is strongly discouraged
• Acute illness (defined as the presence of moderate or severe signs and symptoms) at the time of randomization
• Moderate or severe nasal congestion that in the investigator's opinion could prevent intranasal delivery of vaccine
• Cohort 1 only: weight ≤ 5th percentile for age on the day of randomization (see APPENDIX D)
• Cohort 2 only: history of low birth weight (ie, <2500 grams at birth) or weight ≤ 5th percentile for age on the day of randomization (see APPENDIX D)
• Any drug therapy (chronic or other) within 7 days prior to randomization or expected receipt through the protocol-specified blood collection 28 days after each study vaccine dosing, except that infrequent use of over-the-counter medications for the symptomatic treatment of common childhood illnesses (eg, pain relievers, decongestants, or cough suppressants) are permitted according to the judgment of the investigator
• Any current or expected receipt of immunosuppressive agents including steroids (≥ 2 mg/kg per day of prednisone or its equivalent, or ≥ 20 mg/day if the subject weighs >10 kg, given daily or on alternate days for ≥ 14 days); children in this category should not receive study vaccine until immunosuppressive agents including corticosteroid therapy have been discontinued for ≥ 30 days; the use of topical steroids is permitted according to the judgment of the investigator
• History of receipt of blood transfusion or expected receipt through 28 days after final study vaccine dosing
• History of receipt of immunoglobulin products or expected receipt through 28 days after final study vaccine dosing
• Receipt of any investigational drug within 60 days prior to randomization or expected receipt through 28 days after final study vaccine dosing
• Receipt of any live virus vaccine (excluding rotavirus vaccine) within 28 days prior to randomization or expected receipt within a 28-day window around any study vaccine dose
• Receipt of any inactivated (eg, non-live) vaccine or rotavirus vaccine within 14 days prior to randomization or expected receipt within a 14-day window around any study vaccine dose
• Known or suspected immunodeficiency, including HIV infection
• Living in the same home or enrolled in the same classroom at day care with infants <6 months of age within 28 days after each dose (only one child per household may be enrolled into the study)
• Contact with pregnant caregiver within 28 days after each dose
• Living in a household with someone who is immunocompromised within 28 days after each dose; the subject should also avoid close contact with immunocompromised individuals for at least 28 days after each study vaccine dosing
• Living in a household with someone who works in the healthcare field and who has direct patient care responsibilities within 28 days after each dose
• Living in a household with someone who is a day care provider or preschool teacher for children <6 months of age within 28 days after each dose
• History of allergic reaction to any component of the study vaccine
• Previous medical history or evidence of an intercurrent or chronic illness that, in the opinion of the investigator, may compromise the safety of the subject
• Known or suspected active or chronic hepatitis infection
• History of medical diagnosis of asthma, reactive airway disease, wheezing requiring medication, bronchoconstriction or treatment with a β2 agonist (eg, albuterol), cystic fibrosis, chronic lung disease of prematurity (eg, bronchopulmonary dysplasia), chronic pulmonary disease, medically confirmed apnea, hospitalization for respiratory illness, or mechanical ventilation for respiratory illness (excludes elective mechanical ventilation during surgery for subjects in Cohort 1)
• Immediate family member or household member who is an employee of the Clinical study site or who is otherwise involved with the conduct of the study
• Any condition that, in the opinion of the investigator, might interfere with study vaccine evaluation